A mixing tool of this kind is already known from U.S. Pat. No. 4,007,920, FIG. 18. This known mixing tool is in the shape of a disc, is rotatable around a central axis, and is provided with several axial through bores, with one of the two sides of the disc being convex. The through bores serve to introduce air adjacent to the upper side of the mixing tool into a liquid adjacent to the lower side of the mixing tool. The mixing effect of this known tool, however, is in need of improvement, since for a thorough mixing of liquid and gas the known mixing tool must rotate for a relatively long time and a large amount of energy is therefore consumed.
Another mixing tool of the type given above is the subject matter of two older, not prepublished proposals of the applicant (EP 0 495 506 A2 and DE 41 13 578 A1). The mixing tool therein is designed as a discus-like disc and has different curvatures on its upper and lower sides. The disc itself is caused to rotate by a drive, so that a pressure difference between the upper and lower sides arises as a result of the Bernoulli effect. As the disc has several axial bores, an axial stream created by the pressure difference occurs between the upper and lower sides. The stream flows through the axial bores, so that an intensive blending of several fluids can take place as a result of the flow from the lower side to the upper side. In addition, the known disc is provided with a knife-sharp peripheral edge to prevent a flow around the disc. At a rotary speed of 3000 to 8000 revolutions per minute and a disc diameter of 42 mm the stream is so strong that cavitation occurs at the peripheral edge of the disc and even gases can be dispersed into the tiniest bubbles and dissolved in fluids, whereby the finest foams, suspensions and emulsions are produced.
Cavitation appearances also occur, for instance, with turbine blades or ship propellers. If a liquid is caused to flow at a high speed, cavities with strong partial vacuums are formed in the liquid. When these cavities implode, pressure thrusts are released, which can cause damage to turbine blades and ship propellers in the form of cavitation erosion or cavitation corrosion.
To be sure, the discs according to the above two older proposals of the applicant have proved themselves in practical application; however, endeavours are being made to further increase their cavitation effect, in order to render the mixing of liquids and/or gases even more rapid and even more thorough.